The present invention relates to a doubling circuit for doubling the frequency of an input signal and, more particularly, to a doubling circuit for adjusting the duty ratio of an output signal thereof automatically and implemented as a digital circuit.
A conventional doubling circuit for the above application has an input matching circuit to which an input signal is applied, a non-linear circuit implemented by a transistor, and an output matching circuit to which a load is connected. The input matching circuit matches the input signal to the input portion of the non-linear circuit while the output matching circuit 14 matches a signal whose frequency is double the frequency of the input signal to the output portion of the non-linear circuit. In operation, the input signal is routed through the input matching circuit to the non-linear circuit to be distorted thereby. As a result, high frequencies which are integral multiples of the frequency of the input signal appear at the output portion of the non-linear circuit. As the output signal of the non-linear circuit is applied to the output matching circuit, the matching circuit selects only the frequency double the frequency of the input signal from the input signal. The resulting output signal of the matching circuit, i.e., a signal whose frequency is double the frequency of the input signal is fed to the load.
As stated above, it has been customary to match the frequency at stages just before and just after the non-linear circuit. This makes it difficult to adjust the duty ratio of the output signal to, for example, 50 percent and, therefore, requires troublesome adjustment every time the circuit is reproduced. Although the non-linear circuit may be implemented by a coil or similar analog element, an analog element is apt to introduce errors in the circuitry due to the scattering among such elements and, moreover, obstructs the LSI configuration of the circuit. In addition, the conventional doubling circuit is not applicable to a digital configuration unless a signal having a sinusoidal waveform is transformed to a signal having a rectangular waveform.